#include <iostream>
#include <stdio.h>

using namespace std;

int main(){

	double x;
	double y;
	cin >> x;
	if (x<5){
		y = -x+2.5;
	}
	else if(x<10){
		y = 2-1.5*(x-3)*(x-3);
	}
	else{
		y = x/2-1.5;
	}
	printf("%.3f",y);

    return 0;
}
